U.S. patent number 10,649,081 [Application Number 15/719,835] was granted by the patent office on 2020-05-12 for spaceborne synthetic aperture radar system and method.
This patent grant is currently assigned to United States of America as represented by the Administrators of NASA. The grantee listed for this patent is United States of America as represented by the Administrator of NASA. Invention is credited to Lynn M. Carter, Temilola E. Fatoyinbo Agueh, Kenneth J. Ranson, Rafael F. Rincon.
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United States Patent |
10,649,081 |
Rincon , et al. |
May 12, 2020 |
Spaceborne synthetic aperture radar system and method
Abstract
The present invention relates to an advanced spaceborne
Synthetic Aperture Radar (SAR) system and method that can provide
high resolution measurements of the Earth or planetary surface,
overcoming limitations in conventional SAR systems, and reduce
development costs. The present invention utilizes advanced and
innovative techniques, such as software defined waveforms, digital
beamforming (DBF) and reconfigurable hardware, to provide radar
capabilities not possible with conventional radar instruments,
while reducing the radar development cost. The SAR system
architecture employs a modular, low power, lightweight design
approach to meet stringent spaceborne radar instrument
requirements. Thus, the present invention can enable feasible Earth
and planetary missions that address a vast number survey goals,
including the measurement of ecosystem structure and extent,
surface and sub-surface topography, subsurface stratigraphy, soil
freeze-thaw, ice sheet composition and extent, glacier depth, and
surface water, among many others.
Inventors: |
Rincon; Rafael F. (Greenbelt,
MD), Ranson; Kenneth J. (West River, MD), Fatoyinbo
Agueh; Temilola E. (Washington, DC), Carter; Lynn M.
(Greenbelt, MD) |
Applicant: |
Name |
City |
State |
Country |
Type |
United States of America as represented by the Administrator of
NASA |
Washinton |
DC |
US |
|
|
Assignee: |
United States of America as
represented by the Administrators of NASA (Washington,
DC)
|
Family
ID: |
65897692 |
Appl.
No.: |
15/719,835 |
Filed: |
September 29, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20190101639 A1 |
Apr 4, 2019 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01Q
21/065 (20130101); G01S 13/9076 (20190501); H01Q
9/0414 (20130101); H01Q 25/00 (20130101); H01Q
21/0025 (20130101); H01Q 23/00 (20130101); G01S
7/032 (20130101); G01S 13/9023 (20130101); G01S
13/90 (20130101); H01Q 1/288 (20130101); G01S
13/904 (20190501); G01S 13/9047 (20190501) |
Current International
Class: |
G01S
13/90 (20060101); G01S 7/03 (20060101); H01Q
21/06 (20060101); H01Q 9/04 (20060101); H01Q
21/00 (20060101); H01Q 23/00 (20060101); H01Q
25/00 (20060101); H01Q 1/28 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Armand; Marc Anthony
Attorney, Agent or Firm: Edwards; Christopher O. Geurts;
Bryan A.
Government Interests
ORIGIN OF THE INVENTION
The invention described herein was made by an employee of the
United States Government, and may be manufactured or used by or for
the Government for governmental purposes without the payment of any
royalties thereon or therefor.
Claims
What is claimed is:
1. A synthetic aperture radar (SAR) apparatus comprising: a
plurality of instrument panels containing a plurality of panel
Radar Digital Units (RDUs), each of said instrument panels
including a plurality of subarrays containing a plurality of
subarray RDUs; wherein each of said plurality of panel RDUs and
said plurality of subarray RDUs are configured to form transmit and
receive beams, and to perform waveform generation, data
acquisition, and onboard beamforming; a plurality of feed network
modules connected to said plurality of subarrays; a plurality of
digital and radio frequency (RF) transceiver modules disposed on
each of said instrument panels and connected to said plurality of
feed network modules, said RF transceiver modules which enable
transmit and receive signal conditioning; and a plurality of
antenna elements connected to said plurality of transceiver modules
for signal transmission and reception; wherein said plurality of
transceiver modules are used for both horizontal and vertical
polarization channels, and interface both said plurality of
subarrays and said plurality of antenna elements; said plurality of
panel RDUs, said plurality of subarray RDUs, said plurality of
transceiver modules, and said plurality of antenna elements, are
arranged as subarrays of said plurality of instrument panels; said
plurality of subarray RDUs comprises field programmable gate arrays
(FPGAs) in printed circuit boards (PCB) wherein said FGPAs of said
plurality of subarray RDUs comprise: a timing control system; and a
plurality of FPGA data processors and waveform generators which
enable said centralized waveform generation and data acquisition,
wherein each of said plurality of waveform generators residing at
each of said FGPAs of said plurality of subarray RDUs, creates a
trigger signal which is distributed to each of said plurality of
subarrays, and synchronized by a radar pulse repetition frequency
(PRF), such that said plurality of subarrays digitize and process
radar signals, to remove timing disturbances and ambiguities caused
by noise and part tolerances, wherein said plurality of antenna
elements are a plurality of identical wideband and high
polarization antenna elements; and each of said plurality of
antenna elements includes a dual polarized, aperture-coupled
stacked patch antenna, comprising shells of aluminum walls around
each of said plurality of antenna elements, with stacked resonating
disks, and coupled to a plurality of feedline strips, wherein said
plurality of subarrays maintain signal coherence among multiple
elements in both digital and RF domains, and enable centralized
waveform generation and data acquisition.
2. The apparatus of claim 1, wherein said plurality of subarray
RDUs comprises a plurality of Analog-to-Digital Converters (ADCs)
and Digital-to-Analog Converters (DACs).
3. The apparatus of claim 1, wherein said plurality of transceiver
modules are based on PCB designs or Monolithic Microwave Integrated
Circuit (MMIC) technology; and wherein a receive channel of each of
said plurality of transceiver modules is digitized and processed to
digitally beamform said beams with predetermined characteristics in
a receive operation.
4. The apparatus of claim 1, wherein said plurality of antenna
elements include 50 antenna elements per each of said plurality of
instrument panels, formed of 10 subarrays of five antenna elements
each.
5. The apparatus of claim 1, wherein an array of said plurality of
antenna elements operates over a 200 MHz band, centered at 435
MHz.
6. The apparatus of claim 5, wherein said plurality of subarray
RDUs are programmed to control at least one of antenna gain, beam
pointing angle, or sidelobe structure of the SAR apparatus, in
real-time, during transmit and receive operation, and capable of
executing multi-mode radar operation including at least one of
single, dual, or complete polarimetry SAR imaging, multi-lock angle
data collection, simultaneous left and right of track imaging,
selectable resolution and swath width, digital beam steering, beam
pattern control, nadir SAR altimetry, or scatterometry.
7. The apparatus of claim 6, wherein different data types,
including high- or low-resolution polarimetric imaging,
interferometry, altimetry or scatterometry, and SAR types including
single pass interferometric SAR, scatterometry over multiple beams,
and altimetry, Sweep-SAR (Scan on Receive), simultaneous SAR/GNSSR
(Global Navigation Satellite Systems-Reflection), and simultaneous
active/passive SAR, are implemented.
8. The apparatus of claim 7, wherein the SAR apparatus includes
polarimetric SAR, including measuring horizontal
transmit-horizontal receive (HH), vertical transmit-vertical
receive (VV), horizontal transmitvertical receive (HV), and
vertical transmit-horizontal receive (VH) polarizations).
9. The apparatus of claim 8, wherein the SAR apparatus is used for
Earth science measurements, monitoring of the Earth's surface, or
exoplanetary surface and subsurface imaging.
10. A method of utilizing a synthetic aperture radar (SAR)
apparatus comprising: providing a plurality of instrument panels
containing a plurality of panel Radar Digital Units (RDUs), each of
said instrument panels including a plurality of subarrays
containing a plurality of subarray RDUs; wherein each of said
plurality of panel RDUs and said plurality of subarray RDUs are
configured to form transmit and receive beams, and to perform
waveform generation, data acquisition, and onboard beamforming;
connecting a plurality of feed network modules to said plurality of
subarrays; providing a plurality of digital and radio frequency
(RF) transceiver modules disposed on each of said instrument panels
and connected to said plurality of feed network modules, said RF
transceiver modules which enable transmit and receive signal
conditioning; and connecting a plurality of antenna elements to
said plurality of transceiver modules for signal transmission and
reception; wherein said plurality of transceiver modules are used
for both horizontal and vertical polarization channels, and
interface both said plurality of subarrays and said plurality of
antenna elements; said plurality of panel RDUs, said plurality of
subarray RDUs, said plurality of transceiver modules, and said
plurality of antenna elements, are arranged as subarrays of said
plurality of instrument panels, said plurality of subarray RDUs
comprising field programmable gate arrays (FPGAs) in printed
circuit boards (PCB), wherein said plurality of transceiver modules
are based on PCB designs or Monolithic Microwave Integrated Circuit
(MMIC) technology, the method further comprising: transmitting
beamforming with predetermined beam characteristics using
software-defined waveforms at each of said plurality of subarrays;
digitizing and processing each said receive channel of said
plurality of transceiver modules to digitally beamform said beams
with predetermined characteristics in a receive operation, wherein
said plurality of subarrays maintain signal coherence among
multiple elements in both digital and RF domains, and enable
centralized waveform generation and data acquisition, further
comprising: providing a timing control system; providing a
plurality of FPGA data processors and waveform generators which
enable said centralized waveform generation and data acquisition,
as part of said FGPAs of said plurality of subarray RDUs, the
method further comprising: creating a trigger signal using each of
said waveform generators residing at each of said plurality of
FGPAs of said plurality of subarray RDUs; and distributing said
trigger signal to each of said plurality of subarrays, and
synchronizing said plurality of subarrays by a radar pulse
repetition frequency (PRF), such that said plurality of subarrays
digitize and process radar signals, to remove timing disturbances
and ambiguities caused by noise and part tolerances, wherein said
plurality of antenna elements are a plurality of identical wideband
and high polarization antenna elements, wherein said plurality of
antenna elements include 50 antenna elements per each of said
plurality of instrument panels, formed of 10 subarrays of five
antenna elements each, wherein each of said plurality of antenna
elements includes a dual polarized, aperture-coupled stacked patch
antenna, comprising shells of aluminum walls around each of said
plurality of antenna elements, with stacked resonating disks, and
coupled to a plurality of feedline strips.
11. The method of claim 10, wherein said plurality of subarray RDUs
comprises a plurality of Analog-to-Digital Converters (ADCs) and
Digital-to-Analog Converters (DACs).
12. The method of claim 10, wherein an array of said plurality of
antenna elements operates over a 200 MHz band, centered at 435
MHz.
13. The method of claim 12, wherein said plurality of subarray RDUs
are programmed to control at least one of antenna gain, beam
pointing angle, or sidelobe structure of the SAR apparatus, in
real-time, during transmit and receive operation, and capable of
executing multi-mode radar operation including at least one of
single, dual, or complete polarimetry SAR imaging, multi-lock angle
data collection, simultaneous left and right of track imaging,
selectable resolution and swath width, digital beam steering, beam
pattern control, nadir SAR altimetry, or scatterometry.
14. The method of claim 13, wherein different data types, including
high- or low-resolution polarimetric imaging, interferometry,
altimetry or scatterometry, and SAR types including single pass
interferometric SAR, scatterometry over multiple beams, and
altimetry, Sweep-SAR (Scan on Receive), simultaneous SAR/GNSSR
(Global Navigation Satellite Systems-Reflection), and simultaneous
active/passive SAR, are implemented.
15. The method of claim 14, wherein the SAR apparatus includes
polarimetric SAR, including measuring horizontal
transmit-horizontal receive (HH), vertical transmit-vertical
receive (VV), horizontal transmitvertical receive (HV), and
vertical transmit-horizontal receive (VH) polarizations).
16. The method of claim 15, wherein the SAR apparatus is used for
Earth science measurements, monitoring of the Earth's surface, or
exoplanetary surface and subsurface imaging.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an advanced spaceborne Synthetic
Aperture Radar (SAR) system and method, that can provide high
resolution measurements of the Earth's or a planetary body's
surface, overcoming limitations inherent in conventional SAR
systems, and reducing the development cost. The present invention
is applicable to a number of science and commercial applications
areas including the measurement of ecosystem structure and extent,
surface and sub-surface topography, soil freeze-thaw, ice sheet
composition and extent, glacier depth, and surface water, among
many others.
2. Description of the Related Art
Synthetic-aperture radar (SAR) is a type of radar which creates
two- or three-dimensional images of objects, such as landscapes,
using the motion of a radar antenna over a target region, to
provide finer spatial resolution than conventional beam-scanning
radars. SAR is typically mounted on a moving platform, such as an
aircraft or spacecraft. The distance the SAR system travels over a
target in the time taken for the radar pulses to return to the
antenna creates the large "synthetic" antenna aperture (i.e., 10
Km). Typically, the larger the aperture, the higher the image
spatial resolution, regardless of whether the aperture is physical
(a large antenna) or "synthetic" (a moving antenna), which allows
SAR to create high-resolution images with comparatively small
physical antennas.
In operation, successive pulses of radio waves are transmitted to
"illuminate" a target scene, and the echo of each pulse is received
and recorded. As the SAR device moves, the antenna location
relative to the target changes with time. Signal processing of the
successive recorded radar echoes allows the combining of the
recordings from these multiple antenna positions, which forms the
"synthetic antenna aperture" and allows the creation of
higher-resolution images than would otherwise be possible with a
given physical antenna.
A number of spaceborne SAR instruments are being operated today,
but suffer from the disadvantages of limited coverage and
capability, as well as a high development cost. Thus, a SAR system
which addresses these deficiencies by providing advanced radar
architectures and capabilities beyond conventional SAR systems, and
which enhance the radar capabilities while reducing the cost of SAR
mission, are needed.
SUMMARY OF THE INVENTION
The present invention relates to an advanced spaceborne Synthetic
Aperture Radar (SAR) system and method, that can provide high
resolution measurements of the Earth's or a planetary body's
surface, overcoming limitations inherent in conventional SAR
systems, and reducing the development cost. The present invention
is applicable to a number of science and commercial applications
areas including the measurement of ecosystem structure and extent,
surface and sub-surface topography, soil freeze-thaw, ice sheet
composition and extent, glacier depth, and surface water, among
many others.
The present invention relates to a Synthetic Aperture Radar (SAR)
that enables feasible and affordable spaceborne instruments that
meet or exceed science needs by the science and commercial remote
sensing community. The architecture of the present invention
incorporates advancements in radar technology and techniques making
this radar capable of imaging modes not possible with conventional
radars. The present invention radar's innovative architecture is
based on a multi-channel, modular, low power, lightweight design
approach that permits system customization for different mission
scenarios where the orbit parameters vary (e.g., missions to Earth,
the Moon or Mars).
In one embodiment, the instrument architecture of the present
invention employs multiple radio frequency (RF) transmit and
receive channels, software defined waveform generation, and onboard
digital beamforming and is fully programmable providing agile
imaging capabilities. In one embodiment, in the transmit operation,
each transmit channel is driven with a software program defined
waveform. The waveforms are designed with predetermined phase and
amplitude such that, in the far field, they generate of one or more
energy beams with specific characteristics (pointing angle,
beamwidth, side lobe levels, null positions, etc.). In the receive
operation, each receive channel is digitized and processed onboard.
The onboard processing conditions and frequency-down-converts the
signals and digitally forms one or more beams with specific
characteristics.
The present invention includes software program-defined
multichannel-waveform generators, multi-channel data processors,
digital beamforming, onboard radar processing, broadband and
high-polarization isolation array antennas, and lightweight/low
power RF hardware designs.
In one embodiment, advanced features of the present invention
include software program-defined beam steering (no phase-shifters,
no moving parts), beam pattern control, imaging both sides of the
track, selectable incidence angles, and selectable range
resolution, an increase in the measurement swath (area) without
degrading the measurement resolution, and the suppression of
ambiguities or localized interference in the receiver signal by
appropriate null-steering of the antenna pattern. In one
embodiment, the antenna gain, beam pointing angle, and sidelobe
structure can be programmed in real-time for specific tasks.
Furthermore, multiple beams can be synthesized on both sides of the
flight-track, as well as nadir, using a single nadir-looking
antenna (no moving parts), thus increasing the coverage area.
In one embodiment, the present invention includes spaceborne
architecture of radar technologies and techniques developed for the
airborne L-band Digital Beamforming SAR and for the P-band
polarimetric and interferometric instruments. In one embodiment,
the SAR instrument of the present invention includes architecture
optimized for P-band operation (435 MHz center frequency). However,
the architecture is also applicable to other long wavelength bands,
in particular L-band (1.26 GHz center frequency).
In one embodiment, the instrument architecture is also fully
polarimetric SAR (measures horizontal transmit-horizontal receive
(HH), vertical transmit-vertical receive (VV), horizontal
transmit-vertical receive (HV), and vertical transmit-horizontal
receive (VH) polarizations). In one exemplary embodiment, the SAR
imaging capability of the present invention, is that the
resolution, swath, and imaging angles can be modified in flight,
providing the necessary agility to perform a variety of imaging
modes after launch. The architecture also permits other radar
operational modes of scientific and commercial interest, besides
SAR, such as nadir altimetry, scatterometry, and passive radar
(reflected signals of opportunity).
In one embodiment, a synthetic aperture radar (SAR) apparatus
includes: a plurality of instrument panels containing a plurality
of panel Radar Digital Units (RDUs), each of the instrument panels
including a plurality of subarrays containing a plurality of
subarray RDUs; wherein each of the plurality of panel RDUs and the
plurality of subarray RDUs are configured to form transmit and
receive beams, and to perform waveform generation, data
acquisition, and onboard beamforming; a plurality of feed network
modules connected to the plurality of subarrays; a plurality of
digital and radio frequency (RF) transceiver modules disposed on
each of the instrument panels and connected to the plurality of
feed network modules, the RF transceiver modules which enable
transmit and receive signal conditioning; and a plurality of
antenna elements connected to the plurality of transceiver modules
for signal transmission and reception; wherein the plurality of
transceiver modules are used for both horizontal and vertical
polarization channels, and interface both the plurality of
subarrays and the plurality of antenna elements.
In one embodiment, the plurality of panel RDUs, the plurality of
subarray RDUs, the plurality of transceiver modules, and the
plurality of antenna elements, are arranged as subarrays of the
plurality of instrument panels.
In one embodiment, the plurality of subarray RDUs includes field
programmable gate arrays (FPGAs) in printed circuit boards
(PCB).
In one embodiment, the plurality of subarray RDUs includes a
plurality of Analog-to-Digital Converters (ADCs) and
Digital-to-Analog Converters (DACs).
In one embodiment, the plurality of transceiver modules is based on
PCB designs or Monolithic Microwave Integrated Circuit (MMIC)
technology; and a receive channel of each of the plurality of
transceiver modules is digitized and processed to digitally
beamform the beams with predetermined characteristics in a receive
operation.
In one embodiment, the plurality of subarrays maintains signal
coherence among multiple elements in both digital and RF domains,
and enable centralized waveform generation and data
acquisition.
In one embodiment, the FGPAs of the plurality of subarray RDUs
include: a timing control system; and a plurality of FPGA data
processors and waveform generators which enable the centralized
waveform generation and data acquisition.
In one embodiment, each of the plurality of waveform generators
residing at each of the FGPAs of the plurality of subarray RDUs,
creates a trigger signal which is distributed to each of the
plurality of subarrays, and synchronized by a radar pulse
repetition frequency (PRF), such that the plurality of subarrays
digitizes and processes radar signals, to remove timing
disturbances and ambiguities caused by noise and part
tolerances.
In one embodiment, the plurality of antenna elements is a plurality
of identical wideband and high polarization antenna elements.
In one embodiment, the plurality of antenna elements include 50
antenna elements per each of the plurality of instrument panels,
formed of 10 subarrays of five antenna elements each.
In one embodiment, each of the plurality of antenna elements
includes a dual polarized, aperture-coupled stacked patch antenna,
including shells of aluminum walls around each of the plurality of
antenna elements, with stacked resonating disks, and coupled to a
plurality of feedline strips.
In one embodiment, an array of the plurality of antenna elements
operates over a 200 MHz band, centered at 435 MHz.
In one embodiment, the plurality of subarray RDUs are programmed to
control at least one of antenna gain, beam pointing angle, or
sidelobe structure of the SAR apparatus, in real-time, during
transmit and receive operation, and capable of executing multi-mode
radar operation including at least one of single, dual, or complete
polarimetry SAR imaging, multi-lock angle data collection,
simultaneous left and right of track imaging, selectable resolution
and swath width, digital beam steering, beam pattern control, nadir
SAR altimetry, or scatterometry.
In one embodiment, different data types, including high- or
low-resolution polarimetric imaging, interferometry, altimetry or
scatterometry, and SAR types including single pass interferometric
SAR, scatterometry over multiple beams, and altimetry, Sweep-SAR
(Scan on Receive), simultaneous SAR/GNSSR (Global Navigation
Satellite Systems-Reflection), and simultaneous active/passive SAR,
are implemented.
In one embodiment, the SAR apparatus includes polarimetric SAR,
including measuring horizontal transmit-horizontal receive (HH),
vertical transmit-vertical receive (VV), horizontal
transmit-vertical receive (HV), and vertical transmit-horizontal
receive (VH) polarizations).
In one embodiment, the SAR apparatus is used for Earth science
measurements, monitoring of the Earth's surface, or exoplanetary
surface and subsurface imaging.
In one embodiment, a method of utilizing a synthetic aperture radar
(SAR) apparatus includes: providing a plurality of instrument
panels containing a plurality of panel Radar Digital Units (RDUs),
each of the instrument panels including a plurality of subarrays
containing a plurality of subarray RDUs; wherein each of the
plurality of panel RDUs and the plurality of subarray RDUs are
configured to form transmit and receive beams, and to perform
waveform generation, data acquisition, and onboard beamforming;
connecting a plurality of feed network modules to the plurality of
subarrays; providing a plurality of digital and radio frequency
(RF) transceiver modules disposed on each of the instrument panels
and connected to the plurality of feed network modules, the RF
transceiver modules which enable transmit and receive signal
conditioning; and connecting a plurality of antenna elements to the
plurality of transceiver modules for signal transmission and
reception; wherein the plurality of transceiver modules are used
for both horizontal and vertical polarization channels, and
interface both the plurality of subarrays and the plurality of
antenna elements.
In one embodiment, the method of the present invention further
includes: transmitting beamforming with predetermined beam
characteristics using software-defined waveforms at each of the
plurality of subarrays; and digitizing and processing each receive
channel of the plurality of transceiver modules to digitally
beamform the beams with predetermined characteristics in a receive
operation.
In one embodiment, the method further includes: providing a timing
control system; and providing a plurality of FPGA data processors
and waveform generators which enable the centralized waveform
generation and data acquisition, as part of the FGPAs of the
plurality of subarray RDUs.
In one embodiment, the method further includes: creating a trigger
signal using each of the waveform generators residing at each of
the plurality of FGPAs of the plurality of subarray RDUs; and
distributing the trigger signal to each of the plurality of
subarrays, and synchronizing the plurality of subarrays by a radar
pulse repetition frequency (PRF), such that the plurality of
subarrays digitize and process radar signals, to remove timing
disturbances and ambiguities caused by noise and part
tolerances.
Thus, has been outlined, some features consistent with the present
invention in order that the detailed description thereof that
follows may be better understood, and in order that the present
contribution to the art may be better appreciated. There are, of
course, additional features consistent with the present invention
that will be described below and which will form the subject matter
of the claims appended hereto.
In this respect, before explaining at least one embodiment
consistent with the present invention in detail, it is to be
understood that the invention is not limited in its application to
the details of construction and to the arrangements of the
components set forth in the following description or illustrated in
the drawings. Methods and apparatuses consistent with the present
invention are capable of other embodiments and of being practiced
and carried out in various ways. Also, it is to be understood that
the phraseology and terminology employed herein, as well as the
abstract included below, are for the purpose of description and
should not be regarded as limiting.
As such, those skilled in the art will appreciate that the
conception upon which this disclosure is based may readily be
utilized as a basis for the designing of other structures, methods
and systems for carrying out the several purposes of the present
invention. It is important, therefore, that the claims be regarded
as including such equivalent constructions insofar as they do not
depart from the spirit and scope of the methods and apparatuses
consistent with the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The description of the drawing is only one exemplary embodiment of
the disclosure and not to be considered as limiting in scope.
FIG. 1A shows an instrument array of the SAR system, and FIG. 1B
shows the instrument panel of FIG. 1A, according to one embodiment
consistent with the present invention.
FIG. 2 shows a schematic diagram of the internal architecture of
the SAR system according to one embodiment consistent with the
present invention.
FIG. 3 shows a sub-array system of the internal architecture of the
SAR system, according to one embodiment, consistent with the
present invention.
FIG. 4 shows a schematic layout of the electronics of the SAR
system, according to one embodiment consistent with the present
invention.
FIG. 5A shows the instrument panel of the SAR system, and FIGS. 5B
and 5C show the side view and perspective view of the SAR system
architecture, according to one embodiment consistent with the
present invention.
FIG. 6 shows the multiple types of radar imaging that a SAR system
can implement, according to one embodiment consistent with the
present invention.
DESCRIPTION OF THE INVENTION
The present invention relates to an advanced spaceborne Synthetic
Aperture Radar (SAR) system and method that can provide high
resolution measurements of the Earth or planetary surface,
overcoming limitations in conventional SAR systems, and reduce
development costs. The present invention enables feasible and
affordable spaceborne instruments that meet or exceed science needs
by the science and commercial remote sensing community. The
architecture of the present invention incorporates advancements in
radar technology and techniques making this radar capable of
imaging modes not possible with conventional radars.
The present invention is applicable to a number of science and
commercial applications areas including the measurement of
ecosystem structure and extent, surface and sub-surface topography,
soil freeze-thaw, ice sheet composition and extent, glacier depth,
and surface water, among many others.
The present invention is directed to improving spaceborne SAR
systems by using advanced and innovative techniques, such as
software defined waveforms, digital beamforming (DBF) and
reconfigurable hardware, to provide radar capabilities not possible
with conventional radar instruments, while reducing the radar
development cost. The SAR system innovative architecture of the
present invention employs a novel low power, modular, lightweight
design approach, that allows customization of the instrument
configuration to meet stringent spaceborne radar instrument
requirements for specific mission parameters where the orbit
parameters vary (e.g., missions to Earth, the Moon, or Mars). Thus,
the present invention can enable feasible Earth and planetary
missions that address a vast number survey goals.
The present invention includes software program defined
multichannel-waveform generators, multi-channel data processors,
onboard digital beamforming, onboard radar processing, broadband
and high-polarization isolation array antennas, fully programmable
providing agile imaging capabilities, and lightweight/low power RF
hardware designs.
Specifically, in one embodiment, the instrument architecture of the
present invention employs multiple RF transmit and receive
channels. In the transmit operation, each transmit channel is
driven with a software program defined waveform. The waveforms are
designed with predetermined phase and amplitude such that, in the
far field, they generate one or more energy beams with specific
characteristics (pointing angle, beamwidth, side lobe levels, null
positions, etc.). In the receive operation, each receive channel is
digitized and processed onboard. The onboard processing conditions
and frequency-down-converts the signals and digitally forms one or
more beams with specific characteristics.
In one embodiment, advanced features of the present invention
include software program-defined beam steering (no phase-shifters,
no moving parts), beam pattern control, imaging both sides of the
track, selectable incidence angles, and selectable range
resolution, an increase in the measurement swath (area) without
degrading the measurement resolution, and the suppression of
ambiguities or localized interference in the receiver signal by
appropriate null-steering of the antenna pattern. In one exemplary
embodiment, the antenna gain, beam pointing angle, and sidelobe
structure can be programmed in real-time for specific tasks.
Furthermore, multiple beams can be synthesized on both sides of the
flight-track, as well as nadir, using a single nadir-looking
antenna (no moving parts), thus, increasing the coverage area.
In one exemplary embodiment, the SAR instrument array 100 (see FIG.
1A) of the present invention, which is attached to a spacecraft
101, utilizes a modular approach that distributes the radar
electronics, digital, and antenna subsystems over a plurality of
instrument panels 102 (see FIG. 1A).
In one embodiment, the present invention employs a multiple-input
multiple-output (MIMO) and modular configuration, which distributes
the radar systems into instrument panels composed of "smart" active
subarrays. In one exemplary embodiment, as shown in FIG. 1A, a
plurality of instrument panels, such as a three-panel 103a, 103b,
103c configuration (i.e., Mars' P-band instrument configuration),
includes a plurality of antenna arrays 104 (see FIG. 1B), and
digital and transceiver (RF) modules 105-107 distributed over each
of the plurality of instrument panels 103a-c, that enable SAR
imaging. The actual number of panels is determined by mission
requirements, and the panels are foldable for stowing during launch
and transit to its destination.
In one exemplary embodiment, the full system distribution with each
of the plurality of instrument panels 201 (items 103a-c in FIG.
1A), and which interface with the spacecraft 200, is shown in FIG.
2. In one exemplary embodiment, each of the instrument panels 201
includes a panel radar digital unit (RDU) 201a-c, each interfacing
a plurality of subarrays 203 via a plurality of fiber-optics
interconnects 202. Each of the instrument panel RDUs 201a-c
configures each of the plurality of subarrays 203 and beamforms the
"receive" data. In turn, in one exemplary embodiment, each of the
plurality of RDU subarrays 203 includes a feed network 204 (i.e., a
plurality of feed network modules 204), each connected to a
plurality of transmit receive (T/R) modules 205, and each connected
to a plurality of antenna elements 206. The fiber-optics
interconnects 202 are high speed fiber interconnects 202 for the
data transfer to the instrument panel RDUs 201.
In one exemplary embodiment, the spacecraft RDU 200 interfaces with
the plurality of instrument panel RDUs 201 (e.g., three panel RDUs
201a-c), each of which interfaces with a plurality of subarray RDU
203 units (e.g., ten subarrays 203), which each interface with a
feed network module 204 (e.g., ten feed network modules 204). In
one exemplary embodiment, the feed network modules 204 each
interface with a transmit/receive (T/R) module 205 (e.g., ten T/R
modules 205), and each T/R module interfaces with an antenna unit
206 (e.g., ten antenna units 206).
In one exemplary embodiment, the spacecraft RDU 200 and plurality
of instrument panel RDUs 201 are field programmable gate array
(FPGA) based printed circuit boards (PCB) that employ radiation
tolerant FPGAs. They are designed to be highly compact and power
efficient. The FPGAs of the instrument panel RDUs 201 perform
programming of each instrument panel 201a-c, provide
synchronization, beamform the receive data from each instrument
panel 201a-c, provide housekeeping, and interface with the
spacecraft RDU 200 and the plurality of subarray RDUs 203.
In one exemplary embodiment, FIG. 3 shows a panel architecture 300
view of the plurality of subarray RDUs 301 (item 201 in FIG. 2),
feed networks 304 (item 204 in FIG. 2), transmit/receive (T/R)
modules 305 (item 205 in FIG. 2), and antenna elements 306 (item
206 in FIG. 2). In one exemplary embodiment, each subarray RDU 301
is an FPGA-based PCB, and includes Analog-to-Digital Converters
(ADCs) 302 and Digital-to-Analog Converters (DACs) 303 (see FIG.
3).
In one exemplary embodiment, as shown in FIG. 3, a timing control
system for each subarray RDU 301 works in the background alongside
main FPGA data processors and waveform generators. The timing
system utilizes a software program implemented by a processor
(FPGAs), to maintain signal coherence among multiple elements in
both the RF and digital domains. A high stability central
oscillator provides the frequency reference for the entire
instrument panel 300. Buffering and regeneration at the sub-array
301 and panel 300 levels, is strategically placed for the software
program to maintain adequate signal and jitter levels throughout
the distribution network. High speed ADC clocks are synthesized
locally from the global reference clock using the software
program.
In one exemplary embodiment, radar operations are controlled by a
central trigger signal distributed by the software program to each
sub-array 301 processor, and synchronized by the radar pulse
repetition frequency (PRF). Receive, transmit and calibration
windows are synchronized by the software program, to this trigger.
The FPGA/DAC-based waveform generator residing at each subarray 301
creates the trigger signal and the sub-array 301 FPGA/ADC 302v
digitizes and processes the radar signals, using the software
program, to remove timing disturbances and ambiguities caused by
noise and part tolerances.
Accordingly, the present invention utilizes a distributed digital
electronics architecture that implements advanced waveform
modulation techniques to provide the full beam steering agility
while significantly reducing the system power consumption. The
present invention reduces the number of digital-to-analog
converters (DACs) 303 and analog-to-digital converters (ADCs) 302
(see subarray card 301 of FIG. 3) and enables centralized waveform
generation and data acquisition with reduced power and mass. In one
embodiment, the present invention employs digital beamforming (DBF)
to implement multi-mode radar techniques in a single platform
without slewing the antenna, which is designed to have a reduced
weight.
In one exemplary embodiment, as shown in FIG. 3, the multi-channel
transmit/receive (T/R) radio frequency (RF) modules 305 interface
both the RDU 301 and the antenna elements 306, to condition and
amplify the transmit and the receive signals. Identical T/R modules
305 are used for the horizontal (H) and vertical (V) polarization
channels. In one exemplary embodiment, in the transmit operation,
each transmit channel is driven with software-defined waveforms,
generating one or more beams with specific characteristics
(pointing angle, beamwidth, side lobe levels, null positions,
etc.). In the receive operation, each receive channel is digitized
and processed onboard, digitally forming beams with specific
characteristics.
In one exemplary embodiment, a generic T/R module 305 architecture
is shown in detail in FIG. 4. In the transmit path, the signal from
the RDU (item 301 in FIG. 3) is filtered by filter 401, and
amplified by solid-state power amplifiers (SSPA) 402, 403, whose
gain is chosen to meet requirements. The signal then passes through
the circulator 404 and coupler 405, for calibration, before
reaching the antenna element (item 306 in FIG. 3).
In one exemplary embodiment, in the receive path, the received
signal passes through the same coupler 405 but returns in the
receive path through the circulator 404 (see FIG. 4). The signal is
then filtered by filter 406 and amplified by a low noise amplifier
407, through a switch 408 for additional isolation, then through
two additional amplification stages 409, 410 before passing through
the coupler 411 and back to the processor (FPGA of subarray 301 in
FIG. 3). The total peak transmit power is shared among the modules,
with each transmitting a fraction of the peak power.
In one exemplary embodiment, the design of the transmit/receive
(T/R) RF module 305 (see FIG. 3) of the present invention, utilizes
compact and high efficient PCB designs or Monolithic Microwave
Integrated Circuit (MMIC) technology. In MMIC technology, dense
multi-layer active and passive circuitry are fabricated together on
a semi-insulated semiconductor substrate. The technology thereby
enables the design of radio frequency circuitry with a massive
reduction in mass and volume while enabling multi-function and mass
production. In addition, MMIC technology also increases the
reliability of the design by reducing the number of interconnects
and reducing the part-to-part variation. The T/R module 305 (see
FIG. 3) also includes external connections to the digital
circuitry, bias and control supplies, and transition to the antenna
element 306.
In one exemplary embodiment, the antenna subsystem is made up of
identical wideband and high polarization antenna elements 306. The
antenna array features a low profile panel, which forms one of the
segments of a deployable larger antenna. In one exemplary
embodiment, the antenna array operates over a 200 MHz band,
centered at 435 MHz. In one exemplary embodiment, the antenna array
500 includes a plurality of antenna elements 501 (e.g. 50 antenna
elements) (items 102 in FIG. 1B) per instrument panel (panel 102 in
FIG. 1B), forming ten subarrays of five antenna elements 501 each,
in an exemplary 3.5 m.times.2.62 m panel construction (see FIG.
5A). In one exemplary embodiment, each antenna element 501 includes
a dual polarized, aperture-coupled stacked patch antenna 502 (see
FIG. 5B). In this exemplary embodiment, the two orthogonal
polarizations are excited via two input ports connected to two
feedline strips 503 that are electromagnetically coupled to a
crossed slot cavity-backed 504 aperture in the ground plane (see
FIG. 5C). The aperture is in turn electromagnetically coupled to
the two resonating disks or patches 502a, 502b situated directly
above it.
In one exemplary embodiment, the antenna elements 501 are made of
thin aluminum sheet parts, with the stacked resonating disks 502
and coupling feedline strips 503 supported by dielectric standoffs
and spacers 506 (see FIG. 5). In one embodiment, all the surfaces
are curved into slightly non-planar shapes for increased stiffness,
which ensures higher mechanical resonance frequencies, while
minimizing weight. For smooth beam steering operation, mutual
coupling variations and effects are minimized using thin aluminum
walls 505 around each antenna element 501, forming tray-like shells
505 together with the ground plane sections of each element 501. A
supporting aluminum honeycomb panel and attachment structures
supports all 50 antenna elements 501 in the panel 500 as well as
the T/R modules and RDU PCBs (see FIG. 1B and FIG. 3).
The present invention synthesizes multiple antenna beams,
simultaneously or interleaved, permitting the implementation of
non-conventional imaging that can overcome fundamental limitations
of conventional radar systems. The advantages of the present
invention include an increase in the measurement swath without
reducing the received antenna gain, and the suppression of
ambiguities or localized interference in the receiver signal by
appropriate null-steering of the antenna pattern.
In one exemplary embodiment, the radar architecture is fully
programmable and capable of multi-mode radar operation including
polarimetric synthetic aperture radar (SAR) imaging, nadir SAR
altimetry, and scatterometry (see FIG. 6). In one exemplary
embodiment, the antenna gain, beam pointing angle, and sidelobe
structure can be programmed in real-time for specific tasks.
Further, multiple beams can be synthesized on both sides of the
flight-track, as well as the nadir, using a single nadir-looking
antenna, thus, increasing the coverage area.
In one embodiment, the present invention includes advance
programmable features such as: single, dual, or full polarimetry,
multi-lock angle data collection; simultaneous left and right of
the track imaging; selectable resolution and swath width; digital
beam steering (no moving parts); and beam pattern control, among
others (see FIG. 6).
In one embodiment, the present invention includes spaceborne
architecture of radar technologies and techniques developed for the
airborne L-band Digital Beamforming SAR and for the P-band
polarimetric and interferometric instruments. In one embodiment,
the SAR instrument of the present invention includes architecture
optimized for P-band operation (435 MHz center frequency). However,
the architecture is also applicable to other long wavelength bands,
in particular L-band (1.26 GHz center frequency). In one
embodiment, the present invention utilizes a novel P-band (70 cm in
wavelength) SAR instrument, which is characterized by full
polarimetry, high resolution (<6 m), and programmable beams.
In one embodiment, the instrument architecture is also fully
polarimetric SAR (measures horizontal transmit-horizontal receive
(HH), vertical transmit-vertical receive (VV), horizontal
transmit-vertical receive (HV), and vertical transmit-horizontal
receive (VH) polarizations).
In one exemplary embodiment, the SAR imaging capability of the
present invention, is that the resolution, swath, and imaging
angles can be modified in flight, providing the necessary agility
to perform a variety of imaging modes after launch. In one
exemplary embodiment, the present invention allows for smart data
collection, where a single radar system can provide different data
types and other radar operational modes of scientific and
commercial interest besides SAR, such as high- or low-resolution
polarimetric imaging, interferometry, nadir altimetry or
scatterometry, and passive radar (reflected signals of
opportunity), depending on the requirements defined for each
surface target. For example, several embodiments of synthetic
aperture radar (SAR) include single pass interferometric SAR,
scatterometry over multiple beams, and altimetry. Other techniques,
including Sweep-SAR, simultaneous SAR/GNSSR (Global Navigation
Satellite Systems-Reflection), and simultaneous active/passive, can
be readily implemented providing great enhancements to the present
invention.
The ability of the present invention to rapidly image large areas
of the surface using the simultaneous left/right imaging, with no
degradation in resolution, allows for a fast performance while
still producing full coverage mapping. Alternatively, the present
invention allows for a more efficient use of time as the radar
instrument trades off operations with other selected instruments.
This capability reduces costs and allows for increased spatial
coverage.
The present invention is useful for Earth science measurements such
as the measurement of biomass and ecosystem structure, permafrost,
and soil moisture. A number of other applications range from
monitoring of crops and forests, stocks, to subsurface imaging for
the identification of buried objects or archeological surveys.
In one embodiment, the present invention provides fine resolution
views of subsurface stratigraphy, and includes the ability to
expose bedrock and search for buried features that reveal geologic
history; for example, locating habitable regions, finding water,
and determining planetary hydrology and cryosphere evolution. The
long wavelength signals of the present invention penetrate through
meters of material, images buried surfaces at fine spatial
resolution and full polarimetry, and identify signatures of buried
ice and water. In one exemplary embodiment, the present invention
images through meters of surface-covered regolith and provides
information to characterize the near-surface stratigraphy and
geology. The present invention's fine resolution mapping and
polarimetry would also provide important information on volcanic
processes and lava tubes.
The present invention is also applicable to exoplanetary
explorations missions to image the surface and subsurface of moons
and planets.
It should be emphasized that the above-described embodiments of the
invention are merely possible examples of implementations set forth
for a clear understanding of the principles of the invention.
Variations and modifications may be made to the above-described
embodiments of the invention without departing from the spirit and
principles of the invention. All such modifications and variations
are intended to be included herein within the scope of the
invention and protected by the following claims.
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